Wire providing microwave/radar frequency attenuation is referred to in the wire and cable trade as "filter line." The measurement of the attenuation (insertion loss) upon a given wire's performance relates to the effect that the filter line has upon interference signals conducted down the wire.
Properly shielded filter line provides protection against radiated EMI. Noise currents and voltages are induced on the conductors of the cables when a radiated field causes interference. Filter line can attenuate such noise when it is shielded by metallic braid or other forms of conventional shield layering. The shielding effect can be measured by transfer impedance techniques. the efficacy of filter line wire or cable by providing such shielded wire or cable with additional conductive layers of insulation, such as (1) a jacket of polymeric material that is conductive by reason of high loadings of carbon black; and (2) an additional polymeric layer of insulation containing ferrite particles disposed below the metallic mesh shielding layer.
The high loadings of carbon black are in the range of approximately between 10 wt.% and 35 wt.% of its polymeric matrix. In the past, it has been impossible to extrude such high carbon-filled polymers into insulation for wire and cable. The present invention extrudes the carbon black filled polymeric layer at approximately 580.degree. to 600.degree. F. The higher loaded carbon black polymer is extrudable by virtue of the control of the cross-linking of the carbon black within the polymeric matrix. The polymer, an ethylene-tetrafluoroethylene (ETFE), is mixed with a cross-linking agent, triallylisocyanurate (TAIC), and is additionally radiationally cross-linked. This cross-linking is carefully controlled to allow the carbon black to become part of the polymer matrix, while keeping the viscosity of the crystalline material within extrudable range.
High frequency signals conducted down this wire are partially absorbed by the ferrite particle shield layer. Electromagnetic waves penetrate this shield layer up to the ferrite particles, and are then dissipated by lattice vibration or photon emission.
Protection against radiated EMI is provided by the carbon black of the jacket layer via the percolating structure that consists of large loadings of the carbon black.